There are two main ways in which your friends and family can help you diet.
One method is to get them to go on the diet with you. It is always easier to achieve something when you have a buddy undertaking the same program as you.
You can motivate each other and encourage people as necessary, and the shared determination will get you through.
Alternatively, your friends can help by constantly encouraging you. When you lose weight, in a non-forced way they can let you know how good you are looking; or how nice that new piece of clothing looks on you when you meet your target weight.
These two simple ways friends can help can be the difference between a successful long term program and oscillating back and forth between dieting and not.